Photoelectric conversion devices are used for various kinds of optical sensors, copiers, photoelectric generators, etc. Put into practical use are various types of photoelectric conversion devices, such as those using metals, those using semiconductors, those using organic pigments or dyes, combinations thereof.
“Nature,” Vol. 353, pages 737 to 740 (1991), U.S. Pat. No. 4,927,721, etc. disclose photoelectric conversion devices utilizing fine semiconductor particles sensitized by dyes, and solar cells comprising them. A photoelectric conversion device comprising fine semiconductor particles sensitized by a dye is referred to as a dye-sensitized photoelectric conversion device hereinafter. Electrolytic solutions comprising iodine salts are used in the dye-sensitized photoelectric conversion devices as charge transfer materials. Further known is a photoelectric conversion device in which a molten electrolytic salt comprising an imidazolium salt, a low-melting point compound, is used as a charge transfer material to prevent the leakage and depletion of an electrolytic solution, and thus to improve the durability of the photoelectric conversion device (WO 95/18456). However, a photoelectric conversion device comprising this charge transfer material as a charge transfer layer does not necessarily have sufficiently high photoelectric conversion efficiency. Further improvement of the photoelectric conversion efficiency is thus desired.
It is known that the photoelectric conversion efficiency of a photoelectric conversion device is improved by adding a basic compound such as 4-t-butylpyridine or lithium iodide, etc. to a charge transfer material. However, because a basic compound such as 4-t-butylpyridine is generally a volatile liquid, such a photoelectric conversion device is disadvantageous in durability for a long-term operation. Known as a nonvolatile basic compound usable for the charge transfer material is a polymer having a basic group. However, because such a polymer is solid or liquid at room temperature, a charge transfer material comprising this polymer is highly viscous, resulting in drastic decrease in charge transportability.
On the other hand, known as a means for turning a compound nonvolatile without increasing its molecular weight is a method for introducing a charged group into the compound. For instance, JP 2001-167630 A discloses the use of a compound whose positive-charge group such as an imidazolium group, a pyridinium group, etc. is substituted by a basic group such as a pyridyl group, etc. as a charge transfer material. However, there has been no known example of using a basic compound having negative charge as a charge transfer material. Most of such known basic compounds having charge are not liquid at room temperature, and even if they are liquid, they have extremely high viscosity. Accordingly, their addition to the charge transfer layer results in decrease in charge transportability as in the case of the above polymer. Under these circumstances, it is strongly desired to develop a nonvolatile basic compound, which is a low-viscosity liquid at room temperature.